Rayon manufacture



Patented Jan. 5, 1932 UNITEDSTATES PATENT OFFICE GEORGE A. RICHTER, or BERLIN, NEW HAMPSHIRE, ASSIGNOR TO BROWN COMPANY, OF BERLIN, NEw HAMPSHIRE, A coRroRA'rIoN or MAINE RAYON MANUFACTURE No Drawing.

It is well known that cotton linters or other form of cellulose especially high in alpha cellulose content are suitable for the production of high grade cellulose products, such as rayon. Cot-ton linters, however, be-

ing considerably more expensive than other usable but inferior forms of cellulose, is em ployed to a limited extent only, particularly in the viscose-rayon process.

Heretofore, sulphite pulp has been utilized to some extent for the manufacture of rayon by the viscose-rayon process. The specifications given by the viscose-rayon industry call for sulphite pulp having an alpha cellulose content of about 86.5%, fairly good color, low ash and iron content, and. a viscosity between 3 and 6. Bleached commercial sulphite pulp usually contains about 85% to 87% alpha cellulose, possesses fairly 2 good'color, has a reasonably low ash and iron content, and its processing may be controlled so that its viscosity falls between 3 and 6. The viscosity of the pulp is measured in terms of the viscosityof cuprammonium solution of prescribed pulp concentration prepared therefrom. The viscosity is determined by measuring the time of efiiux of a definite volume under standard conditions through an orifice 'of standard size. The

viscosity of pulp is hereingiven in absolute C. G; S. units, and is determined'by measuring the viscosity or fluidity of a solution of six grams of pulp in a cuprammonium solution composed of 225 cc. or 28 ammonia water containing nine' grams of cuprous oxide. This cuprammonium solution used in determining viscosity value is of arbitrary composition and has been adopted and used in most cellulose laboratories. The C. GJS. viscosity unit is employed because it is definite and denotes a viscosity 100 times that of water at 20 C., wherefore a c'uprammonium cellulose solution of prescribed composition by which a pulp is identified as having a viscosity of 10, is approximately 1,000 times asviscous as water at 20 C. Glycerine which is commonly referred to when discussing viscosiies has a value of between 8 and 9.

The rayon manufacturers have learned from experience that the viscosity of sulphite Application filed January 21, 1927. Serial No. 162,680.

pulp is an indication of its rayon-making qualities For instance, if sulphite pul has, a viscosity below 3, experience has s own .that it is usually overbleached and of low.

alpha cellulose content. Such pulp has poor rayon-making qualities and results in a low strength filament. Similarly, sulphite pulp having a viscosity of 6 or above has usually proven to be unsuitable, as a result of either underdigestion in, its liberation, or underbleaching. Underdigested, it is usually high in non-alpha-cellulose constituent in the form of pentosans, lignin, and other coloring materials. Again, when underbleached it usually contains troublesome residual impurities of this character. Again, when underbleached it usually contains lion-alphacellulose constituent in troublesome form and amount. Xanthation of pulps of this character produces dark-colored cloudy solutions contaming unxanthated fibers which are difficult to remove by filtration and which tend to clog the spinneret, and yielding rayon filaments of low color and strength. It is principally for these reasons that the rayon manufacturers have demanded 'sulphite'pulp having a viscosity falling between 3 and 6. \Vith the foregoing in mind, it was quite natural that the rayon manufactures would reason that any Wood'fiber, in order to be suitable for rayon manufacture, must fall within the viscosity limits found for sulphite pulp. As a matter of fact, they did so reason, and when wood fiber other than sulphite p'ulp wasemployed as a raw material for rayon manufacture, their specifications for such material also called for a viscosity falling between 3 and 6.- Contrary to such a prior reasoning, however, I have discovered that initial viscosity per se does not indicate the suitability of wood fiber for esterification in the manufacture of rayon or other cellulose products, and that it may be advantageous that such fiber possess a viscosity considerably above that specified for-sulphite pulp, for I have found that if a high alpha-cellulose-containing wood fiber of high viscosity is employed as a raw material, rayon or other cellulose products made therefrom are notably 10c Lee conditions of time, temperature and chemical concentration Such digestion effects a solution and removal of-non-alpha-cellulose constituent from the pulp, resulting in alpha fiber which is bleae'hable to high whiteness 1 with only a small quantity of bleach.

Frequently it is desirable to treat the raw pulp prior to alkaline digestion, with a ligninreactive agent, e. g., chlorine water, to dissolve or render soluble its non-alpha-cellulose constituent. Again, where alpha fiber of particularly high alpha cellulose content is desired, the alkaline digestion may be followed by a second alkaline digestion as disclosed in my application, Serial No. 75,888, filed December 16, 1925. Onlya very small amount of bleach is required to whiten to high whiteness alpha fiber produced by a double alkaline digestion. Inasmuch as alpha fiber is produced at a unit cost considerably. lower than the cost of cotton fiber or cotton linters, and has characteristics a'pproximat ing those of cotton fiber, it is an excellent substitute for cotton fiber in the manufacture of rayon and other cellulose products.

Heretofore, in producing alpha fiber for the rayon industry, the processing was so controlled as to produce a product having a viscosity which fell between 3 and 6, i. e., a viscosity which the rayon industry specified for sulphite pulp. This may be accomplished by increasing the severity or extent of any one of the chemical treatments to which the raw cellulosic material or the fiber liberated therefrom is subjected. For instance, over-cooking the raw cellulosic material in the sulphite digester, over pretreating the raw pulp with halogenizing or oxidizing agent, over-digesting the pulp in alkaline solution, or over-.

bleaching the redigested pulp, causes a permanent or irreversible lowering of fiber Vimcosity. By varying one or more of these chemical treatments, I have found that fiber viscosity may be closely controlled and definitely varied. Thus, by over-bleaching the fiber after alkaline digestion under controlled conditions, its viscosity may be reduced to a. point heretofore specified for rayon manucal treatmentsits viscosity has been preserved fiber for esterification or the manufacture of facture, even though through previous chemicellulose products such as rayon; that is, the higher the viscosity, the more suitable the fiber. This opinion is borne out or substantiated by the fact that, while two alpha fibers may possess substantially the samecharacteristics as determinable by the usual chem ical analyses-other than viscosity, their suitabilities for the manufacture of rayon or other cellulose products may be markedly different if their viscosities are difierent. Thus, I- have found that high viscosity alpha fiber yields filaments, films and analogous products of notably higher strength and stretch characteristics than similar products prepared from alpha fiber of low viscosity.

While I am unable to account precisely for these results, my theory is that the known chemical treatments which cause a lowering of the fiber viscosity also cause a depolymeri zation or degradation of the alpha cellulose into lower forms of alpha cellulose, i. e., alpha celluloses of lower molecular weight, which are inferior raw materials. Again, inasmuch as alpha cellulose content is determined by the solubility of the fiber in a caustic soda or caustic potash solution of prescribed strength, such a determination per se does not indicate the suitability of a fiber for rayon or like manufactures, as the alpha cellulose or soda-insoluble content of the fiber may exist in different forms. Fiber viscosity, however. indicates the particular form of alpha cellulose present, a high fiber viscosity connoting the presence of alpha celluloses of high molecular weight, and a low fiber viscosity connoting the presence of alpha celluloses of low moleciilar weight.

In accordance with this invention, therefore, the chemical treatments of the raw cellulosic material or the fiber are carried out with reagents under conditions to modify favorably the characteristics of the fiber for rayon or like manufactures, but without unnecessarilylowering fiber viscosity. A specific example of procedure employed to be practised in producing alpha fiber having a viscosity about 10 may be substantially as follows. Chipped wood, e. g., spruce, is digested in an acid sulphite liquor, say, an acid sodium sulphite liquor containing about 1% combined and about 5% free S0 The wood is digested for about 10 hours, at a maximum temperature of about 285 F. The resulting fiber is separated from the spent liquor and is washed and screened as usual. The washed fiber is then treated at a stock density of about. 5% with a halogenizing or oxidizing solution, e. g., with chlorine water,

containing, say, about 1% to 2% chlorine chlorite and about 1% free caustic soda at about room temperature. The free chlorine in the chlorine water selectively chlormates the ligneous matter in the fiber without appreciably affecting its alpha cellulose content, and is dissolved or converted to products soluble in alkaline solution. lVhen the fiber is treated with a hypochlorite solution, the ligneous matter is selectively oxidized, rather than chlorinated, to soluble reaction products. The presence of caustic soda in the hypochlorite solution retards the activity of the hypochlorite and increases the selectivity of reaction on the ligneous matter. After the halogenizing or oxidizing treatment. the fiber is washed and treated at a stock density of about 10% with a solution containing about 7% caustic soda. The fiber suspension is digested, preferably with agitation, for about six hours at atmospheric boiling temperature. The solution dissolves from the fiber the chlorinated or oxidized products of reaction, less-resistant beta and gamma. celluloses, ligneous matter, and other non-alpha cellulose. constituent, so that the resulting fiber is high in alpha cellulose content. The alpha fiber is washed and is then treated at a stock density of about 10% at about room temperature with a solution containing about 2% to 3% sodium bleach, for about 10 hours. The bleached fiber. is washed and then treated with an antichlor solution, as with a solution of sodium bisulphite, to neutralize residual bleach. The'fiber is then washed and after dilution to the proper consistency is run off on a paper machine into so-called drier sheet, in which form the fiber is sold and shipped for the manufacture of rayon or other cellulose products. The resulting fiber possesses approximately the following characteristics Color 104 to 105 Alpha cellulose 94% Viscosity 10 Soda solubility (3% boiling caustic soda) Copper number 1 to 1.5

Ash content Less than .1

Less than 5 having a viscosity in excess of 10 are subject to rather wide variations. For instance, calcium base chemicals maybe employed in lieu concentration of treating chemicals therein, and the temperature of the treatments are subject to change. So far as I am aware, however, the advantages of producing a Wood fiber of high viscosity and the controlling of all the chemical treatments to produce such fiber have not heretofore been recognized or appreciated. Thus, whereas the usual sulphite fiber having a viscosity in excess of 10 cannot successfully be used for rayon manufacture because of the inherent characteristics previously referred to, I have produced alpha fiber having a viscosity as high as 23, and have made excellent. filaments and yarns therefrom. The successful use of a high viscosity alpha fiber is attributable to the fact that the raw pulp is largely freed from nonalpha cellulose constituent, without, however, unnecessary reduction of viscosity. When, on the other hand, such constituent is removed from sulphite pulp by a chemical overtreatment, as by over-bleaching, a marked lowering of its viscosity, with consequent impairment of its rayon-making qualities, takes place.

A high viscosity alpha fiber such as herein produced may be easily converted into a Xantliate solution of a viscosity suitable for spinning, by properly varying the ageing conditions of the soda cellulose from those maintained when sulphite pulp is employed as the raw material. If soda cellulose produced from high viscosity alpha fiber is aged at the .same temperature and for the same period a the soda cellulose produced from sulphite pulp, the xanthate solution made from the aged or depolymerized alpha fiber would be more viscous than the solution made from the aged sulphite fiber. If, however, the soda cellulose prepared from the high viscosity alpha fiber is aged for a longer period of time, or at a more elevated temperature, or both, it may be converted into a xanthate solution having the proper viscosity for spinning. A change of even one or two degrees in the ageingtemperature markedly affects the viscosity of the final xanthate solution.

In carrying out the viscose process of rayon manufacture, drier sheets of alpha fiber are immersed in a caustic soda solution contain-. ing about 17.5 to 18.5% caustic soda, for about one to two hours, at about 20 C., at the end of which time they arecthoroughly soaked with the solution. The sheets are then subjected to pressure until the ratio of the final weight of the sheets to the weight of the original dry fiber is about 25 to 8, this prescribed ratio ensuring the proper caustic soda. content. The sheeted fiber' is then disintegrated into shreds Which are matured or aged under conditions to bring about the desired depolymerization of the alpha cellulose. The ageing may be carried out at 15 to 20 (3., for about 24 to 72 hours.

The ageing of the caustic soda-treated or much as such depolymerization is 510W and controlled, the resulting depolymerized alpha celluloses appear to possess high molecular weight, as distinguished from the alpha cellulose of low molecular weight produced when alpha fiber is subjected to a chemical treatment which effects a rapid depolymerization.

The aged fiber is treated with the necessary amount of carbon bisulphide for conversion into a xanthate solution, which is then ripened or conditioned at about 20 C., to the desired degree, and spun into filaments or sheeted into films.

By using a high viscosity alpha fiber such as herein produced, certain noteworthy'advantages are realized, to wit:

(a) A higher yield of regenerated cellulose results from a unit weight of original fiber.

(6) A greater economy of chemicals is effected because a considerably lower proportion of impurities is treated along with the alpha cellulose.

(0) S'pinningqis more regular, because of the lower content of insoluble material which is likely to cause a clogging of the orifices of the spinneret.

(d) The viscose solution is lighter-colored and clearer, yielding lighter-colored filaments and films.

(e) The regenerated cellulose product has considerably greater strength and stretch' (f) The filaments necessitate considerably less bleach to be whitened to a given degree, and hence retain more of their initial strength.

The advantages hereinbefore mentioned are of paramount importance in actual prac tioe. Consider, for example, the fact that the color of the unbleached rayon filaments prepared from alpha fiber is much lighter than that of filaments prepared from sulphite pulp. Such a characteristic is of considerable importance, as unbleached rayon filaments are sold as such, and color constitutes a criterion in fixing its price. So, too, the characteristics of strength and stretch of filaments or films are important criterions in fixing the price of such products. 7

While in the manufacture of rayon a definite' viscosity of xanthate solution is neces sary in order to permit successful extrusion through the spinneret, nevertheless for the manufacture of other cellulose products, it may be desirable to produce more viscous solutions of cellulose derivatives other than the xanthate, or to age the caustic-soda-treated fiber under conditions resulting in a depolymerized product from which more viscous Xanthate solutions may be prepared. Moreover, such more viscous solutions of cellulose xanthate or other cellulose deriva- 'as disclosed in my application, Serial No.

122,272,fi1ed July 13,1926.

Rayon or other cellulose products made with the fiber of the present invention possesses physical and chemical characteristics comparable to a product made from good cotton fiber or cotton linters. Thus, it possesses strength and stretch markedly superior to a product made from sulphite pulp and notably superior to a product made from alpha fiber of a viscosity below 10.

Having thus described the nature of this invention and described a way of making and using the same, although without attempting to set forth all of the forms in which it may be made or all the modes of its use, what I claim is:

A process of rayon manufacture, which comprises ageing soda cellulose prepared from Wood pulp having substantially the rayon-making properties of cotton and a Viscosity in excess of 10 under conditions to effect suflicient depolymerization for conversion into a cellulose xanthate solution of viscosity suitable for spinning, converting such soda-cellulose into a cellulose xanthate solution suitable for spinning, and spinning such solution into rayon filaments.

In testimony whereof I have affixed my signature.

GEORGE A. RICHTER. 

